Plenum cable is used to carry power through ducts which are used to ventilate, for example, high rise buildings. While a fire occurring in these ducts can be dangerous in its own right, such a conflagration is especially insidious because the smoke and other gases resulting from the fire are transported through the ducts throughout the building, even to parts quite remote from the blaze. In some cases, colorless and odorless gases can invade sleeping quarters housing unsuspecting people.
The cable used in plenum is generally constructed of a metal conductor insulated with a polymeric material. These elements are generally twisted to form a core and are protected by another polymeric sheath or jacket material. In certain cases, added protection is afforded by inserting a wrap between the core and the sheath.
Thermoplastic non halogen polyolefin compounds containing a metal hydrate flame retardant such as magnesium hydroxide or aluminum hydroxide rely on an endothermic heat sink mechanism for flame resistance. Any interference with this mechanism can lead to a significant loss of effectiveness. Dripping, uneven combustion, loss of ash, and premature release of the hydrate are examples of interference, which lower flame resistance.
The art is constantly seeking to reinforce the heat sink mechanism to maintain flame resistance, particularly where duct fires are concerned.